Nitrile oxide compounds are extremely reactive toward multiple bonds in organic compounds and polymers. This high level of reactivity allows for dinitrile oxides to cure (crosslink) olefin-containing polymers under very mild conditions. This invention discloses a process for the synthesis of stable aryl nitrile oxides which comprises the sequential steps of (1) halomethylating a halomethyl group onto a substituted aromatic compound having at least one substituent group selected from the group consisting of alkyl groups, aryl groups, fused aryl groups, alkaryl groups, halogen atoms, alkoxy groups and nitro groups, wherein said halomethyl group is halomethylated onto a position that is ortho to at least one of the substituent groups on the substituted aromatic compound; (2) converting the ortho halomethylated-substituted aromatic compound into an ortho-substituted aromatic aldehyde by reacting the ortho halomethylated-substituted aromatic compound with a salt selected from the group consisting of sodium 2-nitropropane and potassium 2-nitropropane in a lower alcohol solvent; (3) converting the ortho-substituted aromatic aldehyde into an ortho-substituted aromatic oxime by reacting the ortho-substituted aromatic aldehyde with hydroxylamine; and (4) converting the ortho-substituted aromatic oxime into the ortho-substituted aryl nitrile oxide by reacting the ortho-substituted aromatic oxime with an aqueous sodium hypochlorite solution at a temperature which is within the range of about xe2x88x925xc2x0 C. to about 20xc2x0 C.
Most nitrile oxides (RCNO) are little known, short-lived reactive species that are structurally isomeric with isocyanates (RNCO) and cyanates (ROCN). Being reactive, they readily undergo 1,3-dipolar additions with a great variety of multiple bond functional groups. The relative decreasing reactivity of nitrile oxides with these multiple bonds is roughly: Cxe2x95x90S, Nxe2x95x90N, P(V)xe2x95x90C greater than Cxe2x95x90P(III), Cxe2x95x90As, Cxe2x95x90C, Cxe2x95x90N, Cxe2x95x90Se, Bxe2x95x90N greater than Cxe2x89xa1P, Cxe2x89xa1C greater than P(V)xe2x95x90N, Cxe2x89xa1N greater than Cxe2x95x90O. In the absence of these functional groups, most nitrile oxides will readily dimerize to furoxans (1,2,5-oxadiazole-2-oxides).
Nitrile oxides are mild oxidants and will liberate iodine from solutions of potassium iodide. The nitrile oxide function imposes the same type of solubility characteristics on a molecule that a cyano group does. Aromatic nitrile oxides, especially those where the nitrile oxide is flanked by at least one and preferably two ortho groups have been found to be stable compounds. For example, 2,4,6-Trimethylbenznitrile oxide is a stable crystalline solid (m.p. 105-108xc2x0 C.) with two characteristic strong IR absorptions at 2290 cmxe2x88x921 (xe2x80x94Cxe2x89xa1N) and 1334 cmxe2x88x921 (xe2x89xa1Nxe2x80x94O). In the C13 NMR, the carbon in the xe2x80x94CNO group is found at 35.7 ppm compared with 117.5 ppm for the carbon in the corresponding nitrile compound. This implies that the carbon atom carries considerable negative charge whereas the oxygen bears a positive charge. This highly dipolar character explains their high reactivity toward multiple bonds, see K B G Torssell, xe2x80x9cNitrile Oxides, Nitrones and Nitronates in Organic Synthesis,xe2x80x9d New York: VCH Publ, 1988.
The history of using polyfunctional nitrile oxides or their precursors as crosslinking agents for unsaturated polymers appears to go back to at least 1968 when chemists from Hercules were granted U.S. Pat. No. 3,390,204 for this application. In most cases, the polyfunctional nitrile oxide itself was too unstable to use directly. In these cases, the polyfunctional hydroximoyl halides were used as stable precursors. When mixed with unsaturated elastomers and then exposed to a base such as triethylamine, the hydroximoyl halide function immediately dehydrohalogenates to produce the nitrile oxide group which then rapidly crosslinks the elastomer. The resulting unfilled crosslinked elastomers from styrene-butadiene rubber (SBR), polybutadiene rubber (PBd) and natural rubber (NR) were described as hard, tough and substantially insoluble in chloroform. The crosslinking system was also noted not to be affected by air or moisture. Oddly, this patent makes no mention of any actual cured physical properties or potential applications.
Because no practical system evolved from this work, we can assume it had serious drawbacks either in process control, cost, toxicity and/or ultimate physical properties. Obviously, a two-component system requiring both a hydroximoyl halide and an organic base is not desirable, whereas the use of polynitrile oxide compounds alone is limited only to those nitrile oxides whose stability is sufficient to allow their dispersion and reaction with rubber in preference to self-dimerization. As previously mentioned, very few stable nitrile oxides are known even today. What is known about this class of chemicals is that aromatic nitrile oxides with one or two ortho substituents have enhanced chemical stability with regard to dimerization resistance. Unfortunately, the prime precursors for such nitrile oxides are sterically hindered aromatic dialdehydes. Dialdehydes of this type are relatively difficult to prepare in good yields and purity and this lack of a good synthetic method probably accounts for the field""s lack of development.
In 1989, the situation began to change when Russian chemists developed a novel approach to the synthesis of sterically hindered aromatic dialdehydes, see Leonid I Belen""kii et al, Tetrahedron, Vol 49, No 16, pages 3397-3404, 1993, and Russian Patent SU 4,750,502. With this new technique, aromatic hydrocarbons such as mesitylene or durene could be converted (in several steps) into dialdehydes in yields between 58 to 71 percent (based on hydrocarbon). This was a substantial improvement over the known technique of oxidizing bis-(hydroxymethyl) mesitylene to the dialdehyde with lead tetraacetate described by Christoph Grundmann and Reinhard Richter, xe2x80x9cPreparation of the Dinitrile Oxide of Mesitylene,xe2x80x9d The Journal of Organic Chemistry, 33, 476 (1968).
Within a few years, another Russian group revisited the topic of rubber curing with dinitrile oxides. Only this time, they now had the dinitrile oxide of mesitylene (MDNO) readily available because of the new dialdehyde synthesis, see V V Boiko, N D Malaya and L M Klimenko, xe2x80x9cRheological properties of solutions of diene elastomers with the mesitylene dinitrile oxide,xe2x80x9d International Polymer Science and Technology, Vol 20, No 10, T51, 1993. In their initial studies, they investigated the change in viscosity of various elastomer solutions as a function of time, temperature and MDNO concentration. Polymer solutions evaluated were cis-polyisoprene, polybutadiene, SBR and NBR. The relative rate of gelation was PBd greater than SERxe2x89xa1NBR greater than IR. With PBd and two parts of MDNO, solution gelation tot place in 1.5 hours at 25xc2x0 C.; 0.5 hours at 40xc2x0 C. From this work, they concluded that MDNO could be used as an efficient low-temperature vulcanizing agent for a wide variety of diene elastomers. More recent work by the same Russian group using MDNC and various diene elastomers showed the same order of reactivity as measured by Mooney viscosity increases during mixing, see V V Boiko and I V Grinev, xe2x80x9cInfluence of MDNO/Processing Elastomers,xe2x80x9d International Polymer Science and Technology, Vol 22, No 7, T/21, 1995.
The first practical application of this technology appears to be described in Russian Patent SU 1, 825,829-A1 to the Ukranian Textile Industry Research Institute. In this patent, polymethyl-vinyl-siloxane rubber having a M.W. of 500,000 and a molar olefin content of 0.45-0.55 percent was crosslinked with MDNO in ethyl acetate on a fabric. 
The crosslinked fabric treatment produced a durable water and dirt repellent finish. Russian Patent SU 1,824,389-A1 has also been issued to the Ukranian Textile Industry Research Institute for the synthesis of 2,4,5-trimethylbenzene-1,3-dialdehyde as an intermediate to the corresponding dinitrile oxide as a low temperature hardener.
Russian Patent 2,042,664-C1 describes a new synthesis for dialkylbenzene dinitrile oxides and demonstrated their utility for curing rubbers with low levels of unsaturation at low temperature. Whereas, MDNO has its nitrile oxide groups each flanked by two methyl groups, the new dinitrile oxide compounds have each xe2x80x94CNQ group flanked by only one alkyl group. It is not clear, however, whether or not this structural difference would result in different rates of cure since a comparison control with MDNO was not included in the cured rubber physical property data. Nevertheless, a comparison of the room temperature cured properties of several low unsaturation polymers with the same polymers cured with conventional high temperature sulfur or peroxide systems, showed remarkable similarity.
The starting materials for this new synthesis are meta or para-dimethyl- or meta or para-diethylbenzene. The hydrocarbons are treated with specific molar ratios of aqueous formaldehyde solution, hydrochloric acid, sulfuric acid and acetic acid at 70-85xc2x0 C. to prepare bis-(chloromethyl)-dialkylbenzenes (see Milton J Rhoad and Paul J Flory, xe2x80x9cPreparation of Bischloromethylmesitylene,xe2x80x9d Journal of the American Chemical Society, 72, 2216 (1950)). After cooling to 15-25xc2x0 C., crystals of the bis-chloromethyl compounds are then filtered off, washed with water and dried In the next step, the bis-chloromethyl compounds are treated with an aqueous solution of hexamethylenetetramine in acetic acid for 4 hours at 95-100xc2x0 C. The organic products (dialdehydes) were then extracted from the reaction mixture with carbon tetrachloride. After water-washing the carbon tetrachloride solution, an aqueous solution of hydroxylamine hydrochloride was added followed by the addition of an aqueous NaOH solution to generate free hydroxylamine.
The organic layer was then separated after about 1 hour at 25-35xc2x0 C. The aqueous layer was cooled to 20xc2x0 C. and neutralized to pH 7 with HCl to precipitate the dioxime. The dioxime is filtered off, washed with water and dried.
Treatment of the dioxime with acidified sodium hypochlorite solution at xe2x88x9210 to 0xc2x0 C. gave the crystalline dinitrile oxide compound in very good yields.
Although this synthesis is a great improvement over previous methods, it does have several drawbacks such as the use of a chlorinated solvent and multiple extractions. Additionally, the hexamethylenetetramine/acetic acid method for conversion of the chloromethyl group into an aldehyde is not synthetically useful for the conversion of more highly hindered halomethyl groups such as those found in halomethylated mesitylene.
A W van der Made and R H van der Made, xe2x80x9cPreparation of Bromomethylaromatic Compounds,xe2x80x9d The Journal of Organic Chemistry, 58, 1262 (1993) reports that mesitylene and other structurally similar aromatic compounds can be efficiently bis-bromomethylated in high yields using readily available reagents.
The new synthesis technique of this invention does not use chlorinated solvents or hexamethylenetetramine and can readily convert even a relatively hindered halomethyl group to an aldehyde function in high yield. The overall yield from mesitylene to mesitylene dinitrile oxide by the process of this invention is also higher than that achieved by other known procedures (67 percent versus 50 percent).
The stable nitrile oxide compounds synthesized by utilizing the techniques of this invention can be utilized in a wide variety of applications. For instance, stable nitrile oxide compounds can be used in step-growth polymerization, adhesives, polymer modification and curing rubber.
There are relatively few chemical reactions known that can modify a diene rubber selectively and at low temperature the way nitrile oxides can. This almost unique ability offers many interesting possibilities for polymer modification. One such possibility is the use of a mono-nitrile oxide as a coupling agent between rubber and carbon black. Although the mechanism of this interaction is unknown, Japanese chemists demonstrated that diene rubbers containing either isoxazolidine groups (from nitrone addition to rubber) or isoxazoline groups (from nitrile oxide addition to rubber) greatly enhance the cured tensile properties of filled PBd compounds (see K Tada, Y Numata and T Katsumura, xe2x80x9cModified Polybutadiene by 1,3-Diphenylnitrone and Nitrile Oxides,xe2x80x9d Journal of Applied Polymer Science, 15, 117 (1971).
Because dinitrile oxides are extremely reactive toward double bonds, including the carbon-carbon double bonds in rubbers, they can be employed as a curing (crosslinking) agents for rubbers. Dinitrile oxides are especially suitable for curing coatings or xe2x80x9cvulcanizingxe2x80x9d latex because they will cure or crosslink the olefin-containing polymers under very mild conditions, such as at room temperature.
This invention more specifically discloses a process for the synthesis of stable aryl nitrile oxides which comprises the sequential steps of (1) halomethylating a halomethyl group onto a substituted aromatic compound having at least one substituent group selected from the group consisting of alkyl groups, aryl groups, fused aryl groups, alkaryl groups, halogen atoms, alkoxy groups and nitro groups, wherein said halomethyl group is halomethylated onto a position that is ortho to at least one of the substituent groups on the substituted aromatic compound to produce an ortho halomethylated-substituted aromatic compound; (2) converting the ortho halomethylated-substituted aromatic compound into an ortho-substituted aromatic aldehyde by reacting the ortho halomethylated-substituted aromatic compound with a salt selected from the group consisting of sodium 2-nitropropane and potassium 2-nitropropane in a lower alcohol solvent; (3) converting the ortho-substituted aromatic aldehyde into an ortho-substituted aromatic oxime by reacting the ortho-substituted aromatic aldehyde with hydroxylamine; and (4) converting the ortho-substituted aromatic oxime into the ortho-substituted aryl nitrile oxide by reacting the ortho-substituted aromatic oxime with an aqueous sodium hypochlorite solution at a temperature which is within the range of about xe2x88x925xc2x0 C. to about 20xc2x0 C.
In the first step of the process of this invention, a halomethyl group is halomethylated onto a substituted aromatic compound. The substituted aromatic compound will contain at least one alkyl substituent group, aryl substituent group, fused aryl substituent group, alkaryl substituent group, halogen substituent group, alkoxy substituent group or nitro substituent group. In this step, the halomethyl group is halomethylated onto a position that is ortho to at least one of the substituent groups on the substituted aromatic compound. As a specific example, in the case of naphthalene, the halomethyl group is halomethylated onto a position that is ortho to the fused aryl group (benzene ring) as shown below: 
wherein X represents a halogen atom.
A wide variety of substituted aromatic compounds can be utilized as the starting material. These substituted aromatic compounds will contain at least one alkyl, aryl, fused aryl, alkaryl, halogen, alkoxy or nitro substituent group. Some representative examples of substituted aromatic compounds that can be used include: 1,3,5-trimethylbenzene (mesitylene), 1,2,4,5-tetramethylbenzene (durene), toluene, o-xylene, m-xylene, p-xylene, pseudocumene, isodurene, prehnitene, 1,3-dimethyl-5-t-butylbenzene, 1,3,5-triethylbenzene, 1,3,5-tripropylbenzene, naphthalene, anthracene, xcex1-methylnaphthalene, xcex2-methylnaphthalene, phenanthrene, 1,2-benzanthracene, p-dichlorobenzene, o-chlorotoluene, p-chlorotoluene, p-bromotoluene, bromodurene, bromoisodurene, bromoprehnitene, bromomesitylene, 1-chloro-1-mesitylethylene, p-bromoethylbenzene, 1-chloro-1-mesitylpropene, xcex1-chloroisodurene, p-nitrotoluene, 1-nitronaphthalene, 1,4-diethoxybenzene, 1-ethoxy-4-nitronaphthalene, 1,4-dipropoxybenzene, 1,4-dibutoxybenzene, 1,4-diamyloxybenzene, 2,5-dimethoxytoluene, 2,5-dimethoxyoctylbenzene, 2,5-dimethoxybromobenzene, 3,-chloromethyl-1,2-dimethoxybenzene, 4-(xcex2-chloropropyl)-1,2-dimethoxybenzene, 2,3,4-trimethoxybromobenzene, trimethoxy-p-xylene, 5-propyl-1:3-benzodioxole, and the like. Mesitylene and durene are preferred substituted aromatic compounds.
This halomethylation reaction can be carried out by reacting the substituted aromatic compound with a mixture of acetic acid, hydrogen bromide and paraformaldehyde at a temperature which is within the range of about 50xc2x0 C. to about 120xc2x0 C. For instance, one mole of the substituted aromatic compound (mesitylene) can be reacted with about two moles of the hydrogen bromide and about two moles of the paraformaldehyde. It is typically preferred from this reaction to be carried out at a temperature which is within the range of about 80xc2x0 C. to about 95xc2x0 C.
The halomethylation reaction can, in the alternative, be carried out by utilizing the procedure described by Milton J Rhoad and Paul J Flory, xe2x80x9cThe Synthesis of Polymeric Ethers,xe2x80x9d Journal of the American Chemical Society, Vol 72, page 2216 (1950), the teachings of which are incorporated herein by reference in their entirety. In this procedure, 50 grams (0.37 mole) of durene (melting point of 79xc2x0 C. to 81xc2x0 C.) dissolved in 200 ml (1.5 moles) of 40 percent aqueous formaldehyde and 100 ml. of concentrated hydrochloric acid were heated with stirring on a steam bath while a slow stream of hydrogen chloride gas was bubbled through the mixture. After six hours, the oil layer was separated while hot and set aside to cool. The fine white needles of crude product which deposited where collected, leaving the intermediate monochloromethyldurene in solution. The liquor was treated with a fresh formaldehyde-hydrochloric acid mixture as described above and an additional amount of crude product was obtained. A total of six such treatments of the original durene solution yielded 69 grams (80 percent) of crude bis-(chloromethyl)-durene. A single recrystallization from benzene gave 58 grams (67 percent). The product obtained had a melting point of 193-194xc2x0 C.
In the second step of the process of this invention, the ortho halomethylated-substituted aromatic compound is converted into an ortho-substituted aromatic aldehyde by reacting the ortho halomethylated-substituted aromatic compound with a salt of 2-nitropropane. The salt of 2-nitropropane will typically be a sodium or potassium salt. This step is carried out, utilizing a lower alcohol as the solvent. The lower alcohol will typically contain from 1 to 5 carbon atoms. Some representative examples of alcohols that can be employed as the solvent include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol and n-pentyl alcohol. This reaction step will typically be carried out at a temperature that is within the range of about 20xc2x0 C. to about 100xc2x0 C. It is normally preferred for this reaction to be conducted at a temperature which is within the range of about 30xc2x0 C. to 90xc2x0 C., with temperatures in the range of 40xc2x0 C. to 85xc2x0 C. being most preferred.
In the third step of the process of this invention, the ortho-substituted aromatic aldehyde is converted into an ortho-substituted aromatic oxime by reacting the ortho-substituted aromatic aldehyde with hydroxylamine. This reaction step can be carried out in the same reaction vessel in which the ortho-substituted aromatic aldehyde was synthesized or the ortho-substituted aromatic aldehyde can be isolated and reacted with the hydroxylamine in a separate reaction vessel. This reaction step will typically be carried out at a temperature which is within the range of about 20xc2x0 C. to 100xc2x0 C. Reaction temperatures in the range of 30xc2x0 C. to 70xc2x0 C. are preferred, with temperatures within the range of 35xc2x0 C. to 55xc2x0 C. being most preferred.
In the final step of the process of this invention, the ortho-substituted aromatic oxime is converted into an ortho-substituted aromatic nitrile oxide or ortho-substituted aromatic dinitrile oxide by reacting it with an aqueous sodium hypochlorite (bleach) solution. It is normally preferred to utilize a solution that contains from about 4 weight percent to about 6 weight percent sodium hypochlorite. This reaction step will typically be carried out at a temperature which is within the range of about xe2x88x925xc2x0 C. to 20xc2x0 C.
As has been explained, the technique of this invention can be utilized to convert a wide variety of substituted aromatic compounds into stable nitrile oxide compounds, such as ortho-substituted aryl mononitrile oxides and ortho-substituted aryl dinitrile oxides. The following reaction is representative of conversions of a substituted aromatic compound into a stable nitrile oxide that can be carried out utilizing the techniques of this invention: 
wherein each R1 is independently C1-C12-alkyl, F, Cl, Br, I, Oxe2x80x94C1-C,12-alkyl or Sxe2x80x94C1-C12-alkyl; each R0 is a substituent that does not spontaneously react with the nitrile oxide group; each nxe2x80x2 is independently 0, 1 or 2; nxe2x80x3 is an integer greater than 1; each Xxe2x80x2 is independently a bond or a connecting group; and Yxe2x80x2 is a polyvalent radical containing an ether, ester, amide, amine, carbonate, ketone, urethane, arylene or thioether moiety; or each Xxe2x80x2 and Yxe2x80x2 together are a bond connecting the benzene rings.
The following reactions are also representative of the conversion of various types of substituted aromatic compounds into stable nitrile oxides: 
where R1, R2, R3 and R4 are each independently H, R, halo, SH, SR, SOR, SO2R, hydroxy or OR, with the proviso that at least one of R1, R2, R3 and R4 that is adjacent to a nitrile oxide group is not H; R5, R6, R7 and R8 are each independently H, R, halo, Sxe2x80x94H, SR, SOR, SO2R, hydroxy or OR, wherein R is a C1-C12 linear, branched or cyclic alkyl group, preferably a C1-C4 linear or branched alkyl group, more preferably ethyl or methyl; or R5 and R6 or R7 and R8, together with the carbon atoms to which they are attached, form a benzene ring, wherein at least one of R5 or R7 is not H, and at least one of R6 or R8 is not H; i is 2 or 3; m and n are each 0, 1 or 2 and n+mxe2x89xa72, preferably 2 or 3.
The following reactions are also representative of the conversions of various types of substituted aromatic compounds into stable nitrile oxides: 
where R9, R10, R11 and R12 are each independently H, R, halo, SH, SR, SOR, SO2R, hydroxy or OR with the proviso that at least one of R9 and R11 is not H when a nitrile oxide group is adjacent to both R9 and R11 and at least one of R10 and R12 is not H when a nitrile oxide group is adjacent to both R10 and R12; m, p and r are each 0, 1 or 2 and p+rxe2x89xa72; X is CH2, C(R)2, carbonyl, O, S, SO, SO2, NH, SO2NH, SO2NR or NR; t and u are each 0, 1, 2 or 3; and t+uxe2x89xa72; Y is a bond, C2, C(R)2, carbonyl, O, S, SO, SO2, NH, NR, 9,9xe2x80x2-fluoreno or phenylene.
The following reactions are also representative of the conversions of various types of substituted aromatic compounds into stable nitrile oxides: 